Valve is frequently used in a variety of pipelines for conveying the fluid with the solid particles or liquid drops to attain the purpose for controlling the fluid flow by shutting off or regulating movement now. In addition to a closure member, it has one seat ring installed in a recess surrounding valve upstream passageway or two seat rings installed respectively in recesses surrounding valve upstream and downstream passageways used for keeping engagement with exterior surface of the closure member in order to shut off the fluid flow in the valve efficiently. Having flowed into the upstream passageway, the fluid with solid particles or liquid drops will be split into two branches during flowing to the upstream surface of the closure member. One branch of the fluid flows directly through the upstream opening encircled by the closure member and the upstream seat ring to downstream side of the valve unobstructedly. The other branch will change its flow direction after has impacted frontally on the upstream surface of the closure member exposed in the valve upstream passageway and made it eroded, some of it disperses radially pressing against the upstream surface and erodes it tangentially again, upon that it erodes radially the circumferential surface of bore of the upstream seat ring when impinges on it, and then the dispersed fluids converge into two streams flowing round it in opposite directions to erode tangentially it and the upstream surface of the closure member adjacent to it pressing against them during flowing upon them, and both of them flow respectively through the upstream opening to downstream side of the valve at last; the rest of it flows into the upstream opening pressing against the upstream surface of the closure member and eroding tangentially it, too.
Two ways are used in the prior art valve to enhance the capability of the closure member and the related seat ring for resisting erosion, as follows:
One way is to decrease the flow velocity of the fluid with solid particles or liquid drops flowing through the valve. The erosion rate of material increases exponentially when the particle velocity increases, accordingly the way decreasing flow velocity of the fluid in the valve can dramatically reduce the erosion rate of the upstream surface of the closure member and the circumferential surface of bore of the related seat ring, and prolong lifetime of the valve. But the way will diminish the flow rate flowing through the valve.
Another way used widely at present is to increase the surface hardness of eroded material. Both seat ring and closure member are made of erosion resistant alloys or exceptional hardness ceramics, or metal coated with hard alloys or metal ceramics on their surfaces to enhance the capability of the valve for resisting erosion according to the prior art. Based on research, the erosion rate of the material can be reduced markedly when its surface hardness is higher than the particles carried in the fluid. In fact, we do not hope at all that the sealing surfaces of the closure member and related seat ring are eroded by the fluid with the solid particles or liquid drops during opening and closing movement of the closure member, since any indentations or grooves caused by the erosion on their sealing surfaces will affect sealing effectiveness when the valve is at fully closed position, and leakage paths across the sealing surfaces will be eroded into heavy leaking openings for a moment by the particles or liquid drops passing through them. On the other hand, the fluid carries very hard solid particles such as silica in some processes; it is rather difficult for us to find a material which hardness is harder than them up to now. The fabricating cost of the valve, including material cost, hardening treatment charge and machining expense for hardened parts, will rise substantially even if such material could be found. In addition sizes of angle of incident between streamlines of the particles and planes of locations eroded on the upstream surface of the closure member are various while the closure member is moving, this fact also causes us difficultly to find a material which can resist eroding at a variety of angles of incidence. Moreover erosion rate is affected by many complex factors, for example, shape, size and brittleness of the solid particles carried in the fluid, as well as concentration of the particles, consequently the complicated and knotty problem can not be solved only by improving surface hardness of the material.